diff --git a/runtime/doc/map.txt b/runtime/doc/map.txt
index 8080d8245b..8764ecd875 100644
--- a/runtime/doc/map.txt
+++ b/runtime/doc/map.txt
@@ -1450,7 +1450,8 @@ candidates as a Vim List.  Non-string items in the list are ignored.
 
 The function arguments are:
 	ArgLead		the leading portion of the argument currently being
-			completed on
+			completed on; note that this only captures the current
+			space-separated word, even when using "-nargs=1"
 	CmdLine		the entire command line
 	CursorPos	the cursor position in it (byte index)
 The function may use these for determining context.  For the "custom"